It is the first time FBI director James Comey has publicly confirmed the investigation. (Photo: Reuters/Jonathan Ernst)

ABC News

Donald Trump's claim that the Obama administration had "wire-tapped" his Trump Tower headquarters has been thoroughly and publicly debunked by FBI director James Comey, who also publicly confirmed the bureau was investigating possible links between Russia and Mr Trump's associates.

Key points:

James Comey's testimony before Congress is the first time he has publicly commented on suspicions of Russian links to Trump campaign

The FBI director and other officials also denied any wiretap on Trump Tower

NSA director Michael Rogers labelled Trump administration's claims of British complicity in alleged surveillance of his campaign as "nonsense"

In his much-anticipated appearance before a Congressional committee hearing, Mr Comey demolished the President's assertion, made in a tweet on Saturday March 4.

"The FBI has no evidence to support those tweets", Mr Comey revealed.

Although members of Congress who have been privy to classified briefings have made similar declarations, the director's comment was the first public admission from anyone in a privileged position of knowledge within the US intelligence community that the President's claim had no basis in fact.

Mr Comey had clearly anticipated the line of questioning from the most senior Democrat on the House Intelligence Committee, Adam Schiff, and volunteered to the Congressman that the over-arching Department of Justice did not have any evidence to support Mr Trump's allegations either.

Mr Schiff also put to Mr Comey the President's tweeted assertion that the wire-tapping "is McCarthyism!"

"We have no information to support them [the tweets]. I try very hard not to engage in any 'isms'," Mr Comey replied.

Sitting alongside the FBI director was the other significant figure in the US electronic intelligence gathering community, Mike Rogers of the National Security Agency.

Admiral Rogers also spoke frankly of the NSA's frustration with the President's commentary on intelligence matters since his initial tweets in March.

Responding to questions on the President's quip, Admiral Rogers admitted that the President's decision to raise that surveillance "certainly complicates things" with German intelligence partners, but added that he believed "the relationship is strong enough" to withstand any damage.

The Intelligence Committee hearing has also delivered an official, if guarded, acknowledgement by the FBI that it is in the midst of a thorough investigation into Russian involvement in last year's US election, including any coordination by members of the Trump campaign team.

"I have been authorised by the Department of Justice to confirm that the FBI, as part of our counterintelligence mission, is investigating the Russian Government's efforts to interfere in the 2016 Presidential election," Mr Comey said.

"And that includes investigating the nature of any links between individuals associated with the Trump campaign and the Russian Government, and whether there was any coordination between the campaign and Russia's efforts.

"As with any counterintelligence investigation, this will also include an assessment of whether any crimes were committed."

While noting that the investigation was "open, ongoing and is classified" the FBI director refused to say whose conduct was under investigation.

'Leaks are serious matters'

Whenever questioned about the activities of former national security advisor Mike Flynn and other Trump campaign officials including Paul Manafort, Mr Comey offered a stern "no comment".

The admission opens the possibility that figures associated with the Trump campaign team were being examined for their links with Russia, but were not necessarily the subject of intelligence surveillance — and certainly not "wire-tapping", as the President has claimed.

Republican members of the Committee used their questions to the FBI and NSA directors to probe the spate of leaks of top-secret classified intelligence to the US media from around the time the Trump administration took office.

Mr Comey assured Congress that "leaks are serious, serious matters".

Admiral Rogers added that he had "raised this directly with my own workforce over the course of the last few months to remind everyone that part of the ethics of our profession, not just a legal requirement, as intelligence professionals is that we do not engage in this activity".

"I also reminded the men and women of the NSA, if I become aware of any such conduct there is no place for you on this team," he said.

The White House has not responded to any the testimony, but Mr Trump had sought to pre-empt it with an another series of tweets hours before the hearings.

"The real story that Congress, the FBI and all others should be looking into is the leaking of Classified information. Must find leaker now!" the President wrote.

"The Democrats made up and pushed the Russian story as an excuse for running a terrible campaign. Big advantage in Electoral College & lost!"